The Four Caesars
The death of Emperor Claudius Gothicus in 274 A.D. did not fragment the Roman Empire again as some feared, but it restarted the process of Roman infighting between the Empire's generals. A succession of emperors followed and each one was deposed or assassinated in turn. This ended when Diocletian became emperor. Frustrated by the constant overturn and civil wars that made it impossible to govern effectively, Diocletian determined to place effective checks on imperial power, reducing the incentive for rebellion. He did this by splitting the empire into four parts, each one ruled by its own emperor. Formally, the two emperors in the west and east were styled as Augustus and the two emperors in the north and south were styled as Caesar, but all were of equal rank, had equal authority, and were popularly known as the "Four Caesars."
The first four caesars were Diocletian, Maximian, Galerius, and Constantius Chlorus. The four were represented identically in presentation and image, so as to dissuade any idea that a power structure existed between them. The four were also spread apart, with each one claiming a capital near the frontiers so as to better protect the Roman Empire from raiders or foreign powers. Diocletian was content with this powersharing arrangement and after twenty years forced Maximian to resign with him, leaving Galerius and Constantius to invite two new emperors to take their place in the tetrarchy.